1. Field of Invention
The present invention relates generally to thermal energy storage systems. More particularly, the present invention relates to thermal energy storage systems which utilize a combined internal melt and external melt cycle.
2. Description of the Related Art
Thermal energy storage (TES) systems are used to store thermal energy for use at a later time for heating or cooling processes. For example, the use of TES systems enables electricity at off-peak demand hours to be used to freeze ice. The frozen ice may then be melted during peak electricity demand hours to provide cooling capabilities without significant usage of electricity during the peak demand hours. That is, TES systems are typically arranged to use electricity at off-peak energy demand periods to "store" energy for use during peak energy demand periods. As the efficient use of energy becomes more of a concern, the use thermal energy storage (TES) systems is becoming increasingly popular.
Heat exchangers are generally included as a part of a TES system. A heat exchanger may be arranged, for example, such that cooling liquid may be pumped through the heat exchanger to store energy in a thermal energy storage medium. Such "cold" energy storage is accomplished through cooling the thermal energy storage medium. The thermal energy storage medium is typically in the form of either a low-temperature fluid or a solid such as ice, and is in contact with the heat exchanger. After energy is stored in the thermal energy storage medium, at a later time, the thermal energy storage medium is used to provide chilled air for cooling purposes, as will be appreciated by those skilled in the art. For example, the chilled air may be used as a part of an air-conditioning system that is arranged to cool a building.
Melt cycles are used to melt the thermal energy storage medium to provide a cooled fluid that may be used as part of a cooling system. One melt cycle that is often used to melt a thermal energy storage medium is an internal melt cycle. An internal melt cycle involves melting the thermal energy storage medium, e.g., ice, by allowing a heat exchange fluid to come into indirect contact with the ice. By way of example, a heat exchange fluid which is at a higher temperature than the ice may be pumped through pipes, or enclosed pathways, which are in contact with the ice. As the heat exchange fluid is pumped through the pipes, the ice melts, and the heat exchange fluid cools. The cooled heat exchange fluid is then used as part of a cooling system that is associated with the TES system.
Although an internal melt cycle serves the purpose of providing a cooled heat exchange fluid that may be used as part of a cooling system, the use of an internal melt cycle is not always efficient. Specifically, the heat exchange fluid flows through a pipe and, therefore, does not come into direct contact with the ice. Accordingly, the overall heat transfer between the ice and the heat exchange fluid is affected by both the pipe and the space created between the pipe and the ice as the ice melts. Hence, both the rate at which the ice melts, as well as the amount of cooling which can occur in the heat exchange fluid, are affected. Further, the space between the pipe and the ice increases as the ice melts. Thus, both the ability for the ice to melt and the ability for the heat exchange fluid to be cooled decreases. Therefore, the performance of the TES system has the tendency to become more inefficient as the ice melts. That is, the performance of the internal melt cycle decreases during the course of the cycle.
Another melt cycle which is often used to melt a thermal energy storage medium is an external melt cycle. An external melt cycle involves circulating a fluid, which is to be used as part of a cooling system, such that the fluid comes into direct contact with the thermal energy storage medium, which is typically ice. The fluid, which is cooled as the ice melts, as well as run-off from the melted ice, is used as the cooling medium within a cooling system. For an external melt cycle, although the cooling fluid used to melt the ice may be any of a number of different substances, the cooling fluid is typically water.
As the cooling fluid is in direct contact with the ice during an external melt cycle, the fluid is generally at a temperature which is close to the temperature of the ice. As such, the thermal performance of a TES system which uses an external melt cycle is generally better than the performance which is typically achieved with a TES system which uses the internal melt cycle described above. However, in order to use an external melt system, a tank that is used to house the ice must be sized to accommodate the flow of fluid over the ice. As such, less ice may be formed in a tank of a given size.
Further, uniform ice melt is often difficult to achieve in an external melt system. In order to uniformly melt the ice such that consistency and, therefore, efficiency in the thermal performance of the TES system is maintained, high flow rates for the fluid are often required. In addition, a variety of controls and sensors are typically used to detect undesirable ice build-up, e.g., bridging, which often occurs when ice is not uniformly melted. Such controls and sensors are both expensive and difficult to maintain. However, without the controls and sensors, bridging in the ice often causes ice to be non-uniformly and, therefore, inefficiently melted.
The utility of TES systems is often limited by the performance of the TES systems, as well as by the cost of such systems, and the complexity of controls and sensors that are needed to maintain such systems. As the importance of the efficient use of energy increases, the potential use of TES systems also increases. Hence, the ability to provide efficient, relatively inexpensive, and easy to maintain TES systems is desirable. Therefore, what is desired are methods and apparatus for efficiently providing cooled fluid to a cooling system that is a part of an overall TES system.